To ascertain the immunosuppressive role of uteroglobin its effect on the incorporation of H3-thymidine into maternal lymphocytes in a mixed culture with mitomycin-C inactivated blastomeres was assessed. It was found that when rabbit blastomeres were mixed with maternal lymphocytes, heavy incorporation of H3-thymidine into the DNA of lymphocytes occurred suggesting maternal recognition of fetal antigens. However, such stimulation is totally suppressed when blastomeres were treated with uteroglobin and transglutaminase and then added to the lymphocyte cultures. Inhibition of transglutaminase by NPCNU or MDC failed to produce the suppressing effect of uteroglobin. These results suggest: (1) that uteroglobin may be involved in immonosupression in the rabbit uterus during preimplantation stages; (2) uteroglobin may crosslink with part of the H-2 antigen of the embryo, thus masking this antigen from the lymphocytes of the mother and; (3) this crosslinking of uteroglobin and H-2 antigen may be catalyzed by the uterine enzyme transglutaminase.